The present invention relates to a method for contactlessly measuring a relative position of a magnetic field source which produces a magnetic field and a magnetic field sensor in relation to each other. The present invention further also relates to a corresponding displacement sensor. The invention describes an operating principle of a sensor which is based on the Hall effect and which achieves an increase in the sensor output range with a magnet which is simultaneously reduced in size by storing the earlier value when control by the magnetic field is lost.
By means of the method according to the invention, in particular linear movements are intended to be detected and evaluated contactlessly by means of magnetic interaction between one or more permanent magnets and a magnetic sensor based on the Hall effect.
The measurement of linear movements is used, for example, for controlling machine tools, in pneumatics, in automation technology and robotics and in the automotive sector. A contactless detection of movements affords the advantage inter alia of freedom from wear. The optical and magnetic methods are the most widespread among the contactless measurement methods. Whilst the optical methods ensure a very high level of precision owing to the small wavelength of light, magnetic methods are far less sensitive to dirt and damage, in particular in that magnets and sensor components can be completely enclosed in a non-magnetic hermetic casing.
There are marketed by various producers displacement sensor systems in which the position of a displaceable permanent magnet is established by means of a two or three-dimensional Hall sensor.
In order to detect the relative linear movements at a location, two mutually perpendicular magnetic field components are measured and their quotient is evaluated to detect the position. This method has the advantage that, in regions in which a field component assumes an extreme value and therefore does not detect small displacements, the other field component reacts all the more strongly to displacements so that a substantially equally high level of measurement precision is provided in the complete measurement range.
Furthermore, this principle has the advantage that it is comparatively not very sensitive to a change in the absolute magnetic field strength because proportional numbers between the field components are used to detect the position.
European Patent specification EP 0979988 B1 discloses measurement methods for contactless magnetic detection of relative linear movements between permanent magnets and electronic sensors. In order to detect the relative linear movements by means of the electronic sensors, there are detected at a position two mutually perpendicular field components whose quotient is evaluated in order to detect the position.
In a second method variant, the known measurement method can also be carried out in such a manner that, in order to detect the relative linear movements by means of the electronic sensors, there are detected at two locations two mutually perpendicular field components whose quotient is evaluated in order to detect the position.
The published European Patent Application EP2159546 A2 discloses a measurement method for the contactless detection of relative linear movements between a sensor arrangement for detecting two mutually perpendicular magnetic field components (R, A) and a permanent magnet. A two or three-dimensional Hall sensor is used in place of individual sensors for detecting various field components. The quasi linear position measurement line is formed by the function U=y−e+g, where y is the functional relationship of the field components and e and g are predeterminable voltage values. In particular, a quasi linear position measurement line U=f(y) is formed from the output signals of the Hall sensor according to the relationship y=a+b·R/f(c·Rn+d·An), where R is the radial field component, A is the axial field component, U is the measurement voltage and a, b, c, d and n are constant factors.
The published European Patent Application EP1243897 A1 relates to a magnetic displacement sensor which comprises a magnetic field source and a magnetic field sensor which can be displaced relative to each other along a predetermined path. The magnetic field sensor measures two components of the magnetic field produced by the magnetic field source. There is then derived from the measured components a position signal which constitutes the relative position of the magnetic field sensor and magnetic field source. The explanations set out in this publication in respect of the displacement sensor are distinguished in that the establishment of the position signal contains a division of the two measured components of the magnetic field.
However, those known methods have the disadvantage that the magnetic control field becomes very weak at the ends of the measurement range so that the components of the magnetic flux density used to calculate the position assume small values and therefore the signal-to-noise ratio of both values becomes unfavourable for the calculation.
FIG. 1 shows an arrangement in which a Hall sensor 100 is arranged in a fixed position in order to contactlessly detect a linear movement and the magnetic field of a movable permanent magnet 102 is detected. In accordance with the north/south polarisation in the direction of movement of the permanent magnet 102, the magnetic field extending in the direction of movement is subsequently designated to be the magnetic field component Bz and the component extending transversely thereto is subsequently designated By.
FIG. 2 shows the path of the components By and Bz of the magnetic flux density in accordance with the location z at which the permanent magnet 102 is located. The zero position is the position at which the permanent magnet 102 and the sensor 100 are directly opposite each other.
The angle α which can be calculated in accordance with the following equation (1) is generally used as the measurement signal.
                    α        =                  arctan          ⁡                      (                                          B                ⁢                                                                  ⁢                z                                            B                ⁢                                                                  ⁢                y                                      )                                              (        1        )            
The path of the magnitude || of the magnetic flux density is shown in FIG. 3 as a function of the location z. The vector magnitude || of the magnetic flux density is calculated in known manner from the individual components By and Bz in accordance with the following equation (2). Corresponding calculation rules apply as is conventional for the person skilled in the art when using other coordinate systems or when including a third magnetic field component Bx.|{right arrow over (B)}|=√{square root over (By2+Bz2)}  (2)
As illustrated in FIG. 4, the angle α depends comparatively linearly on the position of the permanent magnet 102 up to a given limit value in relation to the Hall sensor 100. The currently measured characteristic line is generally further linearised, as illustrated in FIG. 4 by means of the line α_lin. That linearised line α_lin then forms the output characteristic line of the sensor. FIG. 5 shows the path of the position signal OUT output by the sensor.
Most commercially conventional 3D Hall sensors can be operated only in the presence of a sufficiently powerful magnetic field. If the permanent magnet is located outside the detection range of the sensor, no sensor signal is available any longer.
There are further known arrangements in which a so-called “clamping”, that is to say, omission of the measurement values at the measurement range edge, is carried out. A fixedly predetermined value independent of the current measurement is output in place of the actual measurement values which are no longer reliable. The US patent specification U.S. Pat. No. 6,502,544 B2 describes such a Hall sensor for a throttle valve arrangement in which the sensor signals are set to the lower or upper clamping voltage which constitutes the minimum or maximum possible output voltage of the sensor, respectively.
However, such clamping voltages are not flexible enough for specific technical applications because they are fixedly preset and do not depend on the current measurement value. In particular, such fixedly set clamped measurement values are unsuitable when the sensor loses the magnetic field at the centre of the dynamic range, as occurs, for example, in H-bridge circuits in the automotive sector.